The present invention relates to the general field of holographic memory. More specifically the invention relates to a system and method for holographic storage.
General holographic storage systems are discussed in xe2x80x9cHolographic Memoriesxe2x80x9d, by Demetri Psaltis et. al., Scientific American, November 1995, which is hereby incorporated by reference. Holography is also discussed in the text Holographic Data Storage, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag, which is hereby incorporated by reference. The basic principles of holography involve the recording of an interference pattern formed between two beams of light, referred to as an object beam and a reference beam. The object beam is encoded with data in a two dimensional pattern. The reference beam is used to form the interference pattern with the encoded object beam and is subsequently used to reconstruct the data by illuminating the recorded pattern.
In volume holographic storage, a large number of holograms are stored in the same volume region of a holographic storage medium. There are several well established methods of holographic storage, such as peristrophic multiplexing, angle multiplexing, shift multiplexing, wavelength multiplexing, correlation multiplexing, and phase multiplexing. Volume holography uses a thick recording medium, where the thickness dimension is associated with Bragg selectivity in the movement of the holographic storage medium in shift multiplexing or the angle change in angular multiplexing.
Angle multiplexing is a volume holography method for storing a plurality of images within a single photorefractive medium. Such angle multiplexing is discussed, for example, in xe2x80x9cHolographic Memoriesxe2x80x9d, by Demetri Psaltis et. al., Scientific American, November 1995, and by P. J. van Heerden in, xe2x80x9cTheory of Optical Information Storage In Solids,xe2x80x9d Applied Optics, Vol. 2, No. 4, page 393 (1963). Angle multiplexing generally involves storage of multiple pages of data in the same photorecording medium by altering the angle of the reference beam entering the crystal during storage of each page while maintaining the position of the object beam. The first page of data is recorded. The angle of the reference beam is then increased until the reconstruction of the first page disappears. Then a new page of data is substituted and holographically recorded. This process is repeated for each successive hologram that is recorded. Any of the recorded holograms can be viewed by illuminating the photorecording medium with a reference beam set at the appropriate angle.
Peristrophic multiplexing is also a volume holography method for storing a plurality of images within a single photorefractive medium. Peristrophic multiplexing is discussed in xe2x80x9cVolume Holographic Multiplexing Methodsxe2x80x9d, by G. Barbastathis and D. Psaltis, published in Holographic Data Storage, pages 22-59, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag. Peristrophic multiplexing is also discussed in xe2x80x9cBeam Deflectors and Spatial Light Modulatorsxe2x80x9d, by G. Zhou, F. Mok, and D. Psaltis. Peristrophic multiplexing generally involves rotation of the recording medium or rotation of the object and reference beam about an optical axis normal to the recording medium.
U.S. Pat. No. 5,483,365 entitled xe2x80x9cMethod for Holographic Storage Using Peristrophic Multiplexingxe2x80x9d, which is hereby incorporated by reference, describes a method of increasing hologram density by rotating the recording material comprising a thin-film photopolymer or, equivalently, by rotating beams used to record holograms in the material. During peristrophic multiplexing, the hologram may be physically rotated, with the axis of rotation being perpendicular to the film""s surface every time a new hologram is stored. The rotation shifts the reconstructed image away from the detector, permitting a new hologram to be stored and viewed without interference, and the rotation can also cause the stored hologram to become non-Bragg matched. Peristrophic multiplexing can be combined with other multiplexing techniques such as angle multiplexing to increase the storage density. Thus, using a combination of peristrophic and angle multiplexing, for example, multiple stacks or sets of holograms can be created in the same volume location of a storage medium.
FIG. 7 illustrates the basic components of a prior art system described in U.S. Pat. No. 5,483,365 for implementing peristrophic and angular multiplexing. A beam splitter 10 splits a coherent monochromatic light beam from a laser 20 into a reference beam (R) and a signal beam (S) which are directed and collimated by optics 30 to a selected recording spot 40a in a holographic recording medium 40 such as a thin layer of lithium niobate and/or a photopolymer film. A spatial light modulator (SLM) 50 modulates the signal beam S in accordance with an input image I. A lens 55 of focal length F between the SLM 50 and the recording medium 40 is displaced from both the SLM 50 and the film 40 by its focal length F, as indicated in the drawing. The signal and reference beams S and R produce an interference pattern in the holographic recording medium 40 which is at least semi-permanently recorded therein. In order to read out the recorded hologram, the reference beam R is projected at the same angle to the same recording spot 40a, to produce an output beam O incident on a detector plane or focal plane array 60 through a spatial filter 70 with aperture A. A lens 80 of focal length F between the detector plane 60 and the film 40 is displaced from both the detector plane 60 and the film 40 by its focal length F.
Angular multiplexing is performed by applying a succession of input images to the spatial light modulator 50 while rotating the recording medium 40 about the Y axis through a corresponding succession of angles while the signal and reference beams S and R continue to illuminate the same recording spot 40a. The Y axis is perpendicular to the plane of interaction defined by the reference beam R and the signal beam S. The plane of interaction is defined such that both the reference beam R and the signal beam S lie in the plane of interaction. In addition to rotation of the holographic storage media, angular multiplexing in the past has been implemented by scanning the angle of the reference beam using a rotatable beam deflector used in conjunction with an imaging lens. Such a system is described in Holographic Data Storage, pages 241-257, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag.
Peristrophic multiplexing is performed by applying a succession of input images to the spatial light modulator 50 while rotating the recording medium 40 about any axis that is not perpendicular to the plane of interaction through a corresponding succession of angles. In one preferred embodiment, this rotation is performed about the Z axis lying in the plane of selectivity. Alternatively the laser, beam splitter, and associated optics utilized to generate the reference beam can be rotated rather than the holographic storage media to implement peristrophic multiplexing. When angular and peristrophic multiplexing are combined, the invention is carried out by angularly multiplexing a maximum number of holograms in the selected recording spot 40a, rotating the medium by a predetermined angle about the Z axis (or any axis not perpendicular to the plane of interaction) and then angularly multiplexing another set of holograms in the same spot. This sequence is repeated until a maximum range of peristrophic multiplexing angles (rotation about the Z axis) has been reached. Then, the entire process is carried out at the next recording spot in the medium.
Although the prior art systems offer the ability to implement both angle and peristrophic multiplexing in the storing of a large number of holograms in a holographic storage media, there are disadvantages to existing systems. Mechanical rotation of the holographic storage media about one or more axis requires additional components and adds system complexity. Rotation of the laser, beam splitter, and associated optics utilized to generate the reference beam is difficult and also adds system complexity.
Thus, there has been a need for improvements in the storage of holograms. More specifically, there has been a need for improved systems and methods for implementing both angular and peristrophic multiplexing.
The present invention provides a solution to the needs described above through a system and method for holographic storage.
The present invention provides a system for storing holograms in a holographic storage medium. The system comprises a laser light source, a first beam splitter for splitting a light beam into an object and reference beam, an elliptical reflector with a first and second focal point, a reflector rotatable about a first axis and a second axis, a pattern encoder, and a holographic storage medium. The reflector is located at the first focal point of the elliptical mirror, and the holographic storage medium is located at the second focal point of the elliptical mirror.
The present invention provides a method for recording multiplexed holograms in a holographic storage medium. The method comprises providing a reference beam and a data carrying object beam. The data carrying object beam is directed to a holographic storage medium and the reference beam is directed to a rotatable reflector which is rotatable about a first axis and a second axis. The rotatable reflector is selectively rotated about the first or second axis to a select position. The reference beam is deflected from the rotatable deflector to an elliptical reflector, and the elliptical deflector further deflects the reference beam to the holographic storage medium where it interferes with the data carrying object beam to record a hologram.
The present invention further presents a method for reading multiplexed holograms recorded in a holographic storage medium. The method comprises providing a readout beam and directing the readout beam to a reflector which is rotatable about a first axis and a second axis. The reflector is selectively rotated about the first or second axis to a select position. The readout beam is deflected from the rotatable deflector to an elliptical reflector, and the elliptical deflector further deflects the readout beam to the holographic storage medium where it reconstructs a previously recorded hologram at the select position to produce a reconstruction beam. The reconstruction beam is then directed to a detector.